RFR: JDK-8279513: jdk/javadoc/doclet/testDocletExample/TestDocletExample.java fails after 8278795

[Jonathan Gibbons]

Please review a test-only fix to make a recent new test more robust.

The test was not fundamentally broken, so the essential functionality remains the same. However, it does assume access to the `src/jdk.javadoc` directory, and crashed when that was not available.

The mitigation is two-fold: 

1. introduce and use a new `SnippetUtils.ConfigurationException` that is thrown when the test cannot find the necessary directories.
2. introduce and use 2 new test-suite keywords, `needs-src` `needs-src-jdk_javadoc` that can be used on the jtreg command-line to filter out this test, and any similar tests in the future.

Tested locally, manually, by temporarily renaming key directories, to test the different code paths.  In all cases, if the test is run and any necessary directories are missing, the test _will still fail_, but now with a more useful and specific exception and detail message.

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Commit messages:
 - JDK-8279513: jdk/javadoc/doclet/testDocletExample/TestDocletExample.java fails after 8278795

Changes: https://git.openjdk.java.net/jdk/pull/8796/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8796&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279513
  Stats: 44 lines in 3 files changed: 37 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8796.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8796/head:pull/8796

PR: https://git.openjdk.java.net/jdk/pull/8796